This invention relates to systems and methods for ion implantation of semiconductor wafers or other workpieces and, more particularly, to methods and apparatus for adjusting beam parallelism in ion implanters.
Ion implantation is a standard technique for introducing conductivity-altering impurities into semiconductor wafers. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
Ion implantation systems usually include an ion source for converting a gas or a solid material into a well-defined ion beam. The ion beam is mass analyzed to eliminate undesired ion species, is accelerated to a desired energy and is directed onto a target plane. The beam is distributed over the target area by beam scanning, by target movement or by a combination of beam scanning and target movement. An ion implanter which utilizes a combination of beam scanning and target movement is disclosed in U.S. Pat. No. 4,922,106 issued May 1, 1990 to Berrian et al.
The delivery of a parallel ion beam to the semiconductor wafer is an important requirement in many applications. A parallel ion beam is one which has parallel ion trajectories over the surface of the semiconductor wafer. In cases where the ion beam is scanned, the scanned beam is required to maintain parallelism over the wafer surface. The parallel ion beam prevents channeling of incident ions in the crystal structure of the semiconductor wafer or permits uniform channeling in cases where channeling is desired. Typically, a serial ion implanter is utilized when a high degree of beam parallelism is required.
In one approach, the beam is scanned in one dimension so that it appears to diverge from a point, referred to as the scan origin. The scanned beam then is passed through an ion optical element which performs focusing. The ion optical element converts the diverging ion trajectories to parallel ion trajectories for delivery to the semiconductor wafer. Focusing can be performed with an angle corrector magnet or with an electrostatic lens. The angle correction magnet produces both bending and focusing of the scanned ion beam. Parallelism may be achieved with an electrostatic lens, but energy contamination can be a drawback.
The output ion beam from the angle corrector magnet or other focusing element may be parallel or may be converging or diverging, depending on the parameters of the ion beam and the parameters of the focusing element. When an angle corrector magnet is utilized, parallelism can be adjusted by varying the magnetic field of the angle corrector magnet. The angle corrector magnet typically has a single magnetic field adjustment which varies both parallelism and bend angle, or beam direction. It will be understood that the ion implanter is often required to run a variety of different ion species and ion energies. When the beam parameters are changed, readjustment of the angle corrector magnet is required to restore beam parallelism.
In prior art ion implanters, the angle corrector magnet is typically adjusted so that the ion beam has normal incidence on a wafer plane of the ion implanter end station. However, the angle corrector adjustment which achieves normal incidence on the wafer plane may result in less than optimum parallelism. In particular, an ion beam that is adjusted for normal incidence on the wafer plane may be somewhat diverging or converging. As shown in FIG. 8, the angle corrector magnet is adjusted such that the center ray of ion beam 200 is normal to wafer plane 202. However, when the beam 200 is adjusted to be normal to wafer plane 202, the parallelism of beam 200 may be degraded such that the beam converges or diverges. The lack of parallelism is unacceptable in highly critical applications.
In another approach, the angle corrector magnet is designed for best parallelism under typical conditions, and the ion implanter end station is positioned for normal incidence of the ion beam on the wafer. However, beam parallelism and normal incidence are not maintained over a wide range of beam parameters, and changing the position of the end station is very difficult.
Accordingly, there is a need for improved methods and apparatus for adjusting beam parallelism in ion implanters.
According to a first aspect of the invention, a method is provided for implanting ions into a workpiece. The method comprises the steps of generating an ion beam, adjusting the ion beam for a desired measure of parallelism, measuring a beam direction of the adjusted ion beam, orienting a workpiece at an implant angle referenced to the measured beam direction, and performing an implant with the workpiece oriented at the implant angle.
The step of adjusting the ion beam may comprise adjusting the ion beam for substantially parallel ion trajectories. In general, the beam direction may differ from the beam axis of the ion implanter. The implant angle may be zero degrees, in which case the workpiece is oriented normal to the measured beam direction.
The workpiece may comprise a semiconductor wafer, and the step of orienting the workpiece may comprise tilting the semiconductor wafer at the implant angle referenced to the measured beam direction.
The method may further comprise the step of measuring an angle of non-parallelism of the ion beam. The step of adjusting the ion beam may be based on the measured angle of non-parallelism. The beam direction and the angle of non-parallelism of the ion beam may be measured with a movable beam profiler and one or more beam detectors.
According to another aspect of the invention, apparatus is provided for implanting ions into a workpiece. The apparatus comprises means for generating an ion beam, means for measuring parallelism of the ion beam, means for adjusting the ion beam for a desired parallelism based on the measured parallelism, means for measuring a beam direction of the adjusted ion beam, means for tilting a workpiece at an implant angle referenced to the measured beam direction, and means for performing an implant with the workpiece tilted at the implant angle referenced to the measured beam direction.
According to a further aspect of the invention, apparatus is provided for implanting ions into a workpiece. The apparatus comprises an ion beam generator, an ion optical element for adjusting the ion beam for a desired parallelism, a measuring system for measuring a beam direction of the adjusted ion beam, and a tilt mechanism for tilting a workpiece at an implant angle referenced to the measured beam direction. An implant is performed with the workpiece tilted at the implant angle referenced to the measured beam direction.
The ion optical element may comprise an angle corrector magnet for adjusting the ion beam for substantially parallel ion trajectories. The measuring system may comprise a movable beam profiler and one or more beam detectors. Where the implant angle is zero degrees, the workpiece is tilted normal to the measured beam direction.